In a drafting plotter, paper and pen move in perpendicular directions over a platen. Motion of the paper is controlled by a drive roller and pinch roller scheme. The drive roller is rotated by a servo-controlled motor-encoder unit acting through a timing belt.
In early prototypes, loud screeching noises emanated from the paper due to amplification of noises intrinsic to digital servo control (namely, servo limit-cycle due to quantization of time and space) resulting from the interaction of the motor-control digital servo and a highly resonant spring-mass system comprising the drive shaft (the spring) and the drive roller (the mass).
One approach to solve this problem would be to add damping between the oscillating elements (the drive roller and paper) and mechanical "ground", such as by using very viscous grease in the drive roller bearings. This approach, however, would waste vast amounts of motor power and would not add damping where it was most needed to suppress the oscillations: in mechanical parallel with the drive shaft, i.e., between motor and drive roller.
A second approach would be to encase the drive shaft with a suitably damped material, to overcome both the objections to the first approach. However, there appear to be no known materials suitable for this purpose, especially in light of space constraints.
A third approach would be to change the drive shaft from steel (the present material) to some less resilient (i.e., more damped) material. However, materials having the requisite damping yet retaining steel's low cost, bending stiffness, and bending strength--needed to sustain design loads (such as timing belt side load)--are not readily available.
Fourthly, another physically resonant system could be added to the paper drive assembly, tuned to suppress the natural oscillations of the first system. However, the success of such a strategy depends on very precise tuning, which is theoretically possible but not realistic in light of the real-world variation in the dynamic behavior of real drafting plotters, both from machine to machine, and over the lifetime of a single machine. This variation stems from part and assembly variations in the factory, material non-linearity and property changes versus temperature, humidity, age, etc., and from the range of sizes and types of plot media the device plots on. In addition, because this strategy would increase dynamic complexity of the mechanical system, it would add considerable complexity to the control problem.
Fifthly, the servo controller could be "tuned" to actively suppress this interaction. Again, this scheme is much too complex and sensitive to real-world variations bo be practical.
Sixthly, the inertias of motor and/or drive roller could be greatly reduced, or the drive shaft that links them made much stiffer (such as by increasing its diameter) so that the oscillations of the drive train would become both smaller in amplitude and higher in frequency, enough so that they could no longer meaningfully interact with the digital servo. Extremely low inertia motors and drive rollers are expensive, while constraints on drive shaft diameter and inertia prevent its stiffness from being arbitrarily increased.
Finally, the quantization errors which give rise to acoustic noise could themselves be suppressed. The sample rate of the digital servo and the resolution of the shaft encoder used to provide motor position information could both be increased. These are both very expensive undertakings requiring major architectural changes in the design of a plotter, not realistically available when low cost and easy retrofit to existing designs are required.
Accordingly, a need remains to provide a plotter employing a low cost drive scheme, in the presence of resiliantly compliant drive train components, with means to suppress the transmission and amplification of acoustic noise originating in the motor control servo.